The present invention relates to a feedback control apparatus for an internal combustion engine.
A feedback control apparatus is known in the art in which the oxygen concentration of exhaust gases or the air-fuel ratio is compensated by controlling the amount of additional air supplied to the exhaust gases or the air-fuel ratio of the mixture in accordance with the detection signal of an exhaust gas sensor adapted to detect the air-fuel ratio by detecting the concentration of a particular component of the exhaust gases, such as, an oxygen concentration sensor (O.sub.2 sensor) for detecting the concentration of the oxygen in the exhaust gases.
However, the known feedback control apparatus of this type is disadvantageous in that due to the variations in preset basic air-fuel ratio among different carburetors caused in the course of manufacture, the variations in accuracy among different control components, such as, additional air actuators in apparatus of additional air supply type, the variations in characteristics among circuit electric elements of the same type, etc., if the center of the air-fuel ratio control deviates from the preset value, the air-fuel ratio control will become unstable. This is considered to result from the fact that the control is effected by varying the amount of compensation in accordance with the result of detection whether the air-fuel ratio is on the rich or lean side as compared with the preset control center instead of detecting the amount of deviation of the air-fuel ratio from the preset control center and feedback controlling the air-fuel ratio in response to the amount of correction corresponding to the detected amount of deviation. Thus, the known apparatus is not capable of stably controlling the air-fuel ratio at a desired value if the substantial control center varies.